automake: Prevent `dist` or `distcheck` when uncommitted changes are present

Makefile.am

@@ -19,6 +19,7 @@ EXTRA_DIST = \
 	git-version-gen
 
 dist-hook:
+	@if ! git diff --quiet; then echo "Uncommitted changes present; not releasing"; exit 1; fi
 	echo $(VERSION) > $(distdir)/.tarball-version
 
 docs/html: $(top_builddir)/doxygen.cfg $(top_srcdir)/include/plist/*.h